1. Field of the Invention
The present invention relates to a locking mechanism. More specifically, the present invention relates to a locking mechanism for use in a screw and rod fixation assembly.
2. Description of Related Art
Stabilization of the spine for various conditions, including degenerative disc disease, scoliosis, spondylolithises, and spinal stenosis often requires attaching implants to the spine and then securing the implants to spinal rods. Such spinal fixation devices can immobilize the vertebrae and can alter the alignment of the spine over a large number of vertebrae by means of connecting at least one elongate rod to the sequence of selected vertebrae. Such rods can span a large number of vertebrae, such as three or four. However, the spine anatomy rarely allows for three or more implants to be directly in line. In order to allow for this irregularity, the rod must be contoured to the coronal plane. With anatomical curvature in the saggital plane found in the lumbar spine, the rod has to be contoured in both planes, requiring considerable effort and surgical time.
For example, the U.S. Pat. Nos. 5,554,157, 5,549,608, and 5,586,984, all to Errico et al., disclose polyaxial locking screws and coupling element devices for use with rod fixation apparatus. The '157 patent discloses a coupling element including an interior axial passage having an interior surface which is inwardly curvate at the lower portion thereof such that it comprises a socket for polyaxially retaining a spherical head of a screw. The coupling element further includes a pair of vertically oriented opposing channels extending down from the top of the coupling element which define therebetween a rod receiving seat. The channel further provides the walls of the upper portion to a pair of upwardly extending members, each including an exterior threading disposed on the upper most portion thereof for receiving a locking nut. During the implantation of the assembly, the locking nut seats against the top of the rod which in turn seats on top of the screw head. The nut causes the rod to be locked between the nut and screw and the screw to be locked in the socket.
The '608 patent discloses a modification wherein a locking ring is disposed about the exterior of the lower portion of the coupling element and provides an inward force on an outwardly tapered portion upon downward translation thereof, causing the interior chamber to crush lock a screw head therein to eliminate the polyaxial nature of the screw element coupling.
The '984 patent discloses a polyaxial orthopedic device including a coupling element having a tapered lower portion including a slotted interior chamber in which a curvate head of a screw is initially polyaxially disposed. The coupling element includes a recess for receiving a rod of the implant apparatus. A locking ring is disposed about the lower portion of the coupling element and provides an inward force on the outwardly tapered portion upon downward translation thereof. The vertical slots are caused to close and crush, thereby locking the screw head within the inter chamber thereof.
With regard to a posterial spinal fixation system, the size of the implant is an important issue. Bulky components can cause soft tissue irritation, as well as comprise the facet joints at the end of a fusion. Minimizing the size of the implants used is critical in spinal surgery with regard to the issue of bulky components. Soft tissue irritation resulting from extensions of implants is a common occurrence. Many times it is caused by the implant being too high relative to its environment. For example, implants can be too high to sufficiently be covered with muscle tissue. Hence, a reduction in the overall height of an implant is a critical advantage for the patient.
Most spine implants utilize a rod connection mechanism that requires a nut, connector, or other component to secure the implant to the rod, such as in those prior art patents discussed above. This connection means has at least some portion that sits above the rod to which the implants are connected. This increases the overall height of the implant assembly in the body.
Several attempts have been made in the prior art to address this concern of the overall height of the implant assembly. Modulok™ (manufactured by Zimmer and Wright Medical, Arlington, Tenn.), Versalok™ (manufactured by Wright Medical, Arlington, Tenn.), and Paragon™ (manufactured by DANEK, Memphis, Tenn.) are all products in the marketplace utilizing a taper connection mechanism to minimize the amount of material situated above the rod in the spinal assembly. However, these types of designs have various inherent difficulties. The Modulok™ system requires a tapered sleeve to be snapped in place over the rod prior to locking the rod to the implant. The Paragon™ system requires use of two tapered sleeves. The Versalok™ system, specifically designed for lower back fixation, requires that the rod be snapped into a channel and an outer sleeve be brought up to compress the rod interface.
All of the above-mentioned systems have drawbacks, including the bulky instrumentation needed to engage the tapers which somehow must be brought into the surgical environment, thereby causing various difficulties in surgical use. For example, a surgeon usually determines the contour of a rod by contouring the rod and placing it into the saddle or seat of the implants. Then the surgeon examines the contour, removes the rod, and re-contours the rod as required.
Others have approached the solution to this problem with various poly-axial screw designs. For example, in U.S. Pat. No. 5,466,237 to Byrd et al., a bone screw is described that includes a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and a spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. While this particular approach utilizes a minimum of components, the security of the fixation of the bone screw to the rod is lacking. In other words, the engagement or fixation between the small spherical projection on the bone screw and the spinal rod is readily disrupted when the instrumentation is subjected to the high loads of the spine, particularly in the lumbar region.
In another approach disclosed in U.S. Pat. No. 4,946,458 to Harms et al., a spherical headed bone screw is supported within separate halves of a receiver member. The bottoms of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. In another approach taken by Harms et al., in U.S. Pat. No. 5,207,678, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member are threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts. It is known that threading rods can tend to weaken the rods in the face of severe spinal loads. Moreover, the design of the bone screws in the '458 and '678 patents require a multiplicity of parts and are fairly complicated to achieve complete fixation of the bone screw.
A further approach illustrated in U.S. Pat. No. 5,797,911 to Sherman et al. is to provide a U-shaped holder through the top of which a bone fastener topped with a crown member is loaded. The holder accommodates a rod in a channel above the crown member and a compression member above the rod. The compression member presses on the rod and crown member to lock the fastener against the holder in any of a number of angles in three dimensions with respect to the rod. This approach is somewhat bulky in order to accommodate the other structural components.
Yet a further approach is disclosed in U.S. Pat. No. 5,733,285 to Errico et al., in which a holder is provided with a tapered and colletted portion at the bottom into which a bone fastener head is inserted. A sleeve is provided that slides down around the colletted portion to crush lock the colletted portion around the head of the bone fastener. This apparatus is believed to be relatively bulky and difficult to manipulate given the external sliding locking mechanism. It is further dependent on the fit of the external sleeve and the relative strength of the collet and its bending and crushing portions for secure locking of the bone fastener head.
Another important aspect for improving a polyaxial screw is having a screw head securing means that provides a strong lock while minimizing size. All known polyaxial screws in prior art require substantial size due to the locking mechanism.
For example, U.S. Pat. No. 6,280,442, to Barker et al. discloses a complex locking means having a screw head with complex head geometry, a crown member, and an outer rigid body. Locking occurs by compressing the crown member against the complex head, which compresses the head against the rigid seat. This crushes the machined ridges on the head and secures the screw.
Another example, and one common in many polyaxial screw patents, is the use of some sort of collet that has a spherical seat with a flexible portion that is designed to deflect around the screw. By compressing the flexible portion against a rigid outer wall, the collet is compressed against the head to cause locking. Such configurations are disclosed in U.S. Pat. No. 6,053,917 to Sherman et al., U.S. Pat. No. 6,010,503, to Richelsoph, and U.S. Pat. No. 5,964,760, to Richelsoph. Tartar in U.S. Pat. No. 5,910,142 discloses a spherical collet that is compressed between the screw head and the rod. U.S. Pat. No. 5,891,145 discloses a very complex double wedge locking means. The more complex the locking means the larger the components need to be and the more expensive to manufacture.
In addition, the majority of the locking mechanisms use a set screw, nut, or other threaded locking means. Threaded devices tend to be subject to cross-threading. Therefore, considerable effort must be placed into instrument design and surgeon technique to prevent this from happening. In addition, proper locking can only achieved by exerting a controlled locking force, such as a torque wrench, or a set screw designed to fracture the head portion at the proper load. Furthermore, the exertion of torque on the set screw exerts a force on the implant and bone. This detrimental load on the spine must be countered by another instrument referred to as a counter-torque instrument. This makes the surgical procedure more difficult, as room must he provided for the counter-torque instrument.
There have been other solutions. U.S. Pat. No. 5,683,392, to Richelsoph et al., discloses a non-threaded locking means whereby an outside collar is drawn upwards over an inner collet to lock the screw and rod simultaneously. This mechanism requires considerable force to engage the collar and is bulky due to the wall thickness needed in the collar to maintain compressive strength. U.S. Pat. Nos. 6,010,503 and 6,355,040, to Richelsoph show a collet that is inserted within the body portion of at implant, such that the walls of the collet deflect and bend to engage the rod and lock the assembly. While this creates a low profile, the mechanism is complex and requires significant energy to lock the system together.
The energy required to lock the system further complicates the instrumentation. Instruments become larger and more complex whenever this is an increase in required locking force. This is partly due to the energy loss in locking due to elastic and plastic deformation of the implant and elastic energy losses due to flexing of the instruments.
It would therefore be useful to develop a locking means that locks securely without exerting any torsional load on the spine, requires no torque wrench or breaking set screws, self adjusts to exert proper locking force, is uneffected by in vivo conditions (for example, blood and debris in threads), eliminates threads and the risk of cross-threading, as an efficient use of energy to allow maximum use of the locking energy without frictional or mechanical losses, can stay within known acceptable tolerances for locked components, and is small, compact, and simple.